I just got an HP-15C in a flea market for some $20. Aside from some scratches, it is in good condition. Bought it without batteries, so I installed fresh ones and was ready to go.

The little machine was made in the US in early 1985. I tested it using a favorite of mine, the sine integral. I noted that the time to calculate the integral varies with display format (the more digits, the longer it takes.) A number left in the y register appears to be the uncertainty of approach.

I think I read that calculation time depends on display format. How it depends? Is the algorythm similar to that used on the HP-41 Advantage Pac?